Among the conventionally-known steering apparatus are electric power steering apparatus and steer-by-wire steering systems. As well known, the electric power steering apparatus are steering assisting apparatus which are designed to activate an electric motor (steering assisting motor) as a human operator or driver manually operates the steering wheel, during driving of a motor vehicle, to thereby assist the driver's manual steering effort. In such electric power steering apparatus, the steering assisting motor, which provides a steering torque assist, is controlled by a motor control section (ECU), using a steering torque signal generated by a steering torque detection section detecting steering torque that is produced on the steering shaft by driver's operation of the steering wheel and a vehicle velocity signal generated by a vehicle velocity detection section detecting a traveling velocity of the vehicle, so as to reduce manual steering force to be applied by the human driver. Specifically, to control the steering assisting motor, the motor control section sets a target value of a motor current to be supplied to the motor on the basis of the above-mentioned steering torque signal and vehicle velocity signal, and then it calculates a difference or offset between the thus-set target current value and a value of a motor current detection signal fed back from a motor current detection section that detects a motor current to actually flowing through the motor. Then, the motor control section performs a proportional/integral compensating process (PI control) on an offset signal representative of the calculated offset, to thereby generate a signal for controlling driving of the steering assisting motor.
The electric power steering apparatus have been developed so far primarily for compact vehicles. However, in recent years, there has also arisen a need for large-sized vehicles (e.g., passenger cars with a displacement of 2,000 cc or over) to be provided with electric power steering apparatus, with a view to achieving reduced fuel cost, increased vehicle control range, etc. In the case of large-sized vehicles provided with an electric power steering apparatus having a single steering assisting motor, the steering assisting motor has to be a high-power motor capable of providing a great steering assisting force, due to a vehicle's great weight. Therefore, the size of the steering assisting motor tends to increase, and the increased size would result in lowered flexibility in a mounting layout (lowered mountability) of the motor on the vehicle body. Further, such large-sized vehicles require a non-standardized, dedicated steering assisting motor and a motor control drive section therefor, which would result in increased manufacturing costs. To avoid the inconveniences, there have been proposed, in, for example, Japanese National Laid-Open Publication No. 2001-525292 and Japanese Patent Laid-Open Publication No. 2001-260908 and No. 2001-151125, more sophisticated electric power steering apparatus suitable for large-sized vehicle, which include two steering assisting apparatus.
Generally, the conventional electric power steering apparatus comprise a sensor unit including a steering torque diction section etc. an ECU including a CPU, motor drive circuitry, etc., and an electronic drive control unit including current supply elements for supplying a motor current from the ECU to the steering assisting motor. Further, in case a failure occurs in any of the ECU and motor-driving electronic drive control unit, the conventional electric power steering apparatus illuminate a warning lamp via a display panel disposed in front of a driver's seat or the like on the basis of fail-safe control, and, if the steering force assist control can not be performed completely, the steering apparatus are shifted to a normal steering mode based on driver's manual operation alone.
In recent years, there has been a demand that, even when a failure has occurred as noted above, the electric power steering apparatus be appropriately maintained in operative condition to keep assisting the manual steering effort of the driver. For that purpose, the steering apparatus may be constructed to redundantly include two motor drive circuits so that the apparatus can keep assisting the manual steering effort even when a failure has occurred in the motor-driving electronic drive control unit or the like.
However, such electric power steering apparatus redundantly including two motor drive circuits would present the following inconveniences. It is ordinary today to control the steering assisting motor by means of a PWM (Pulse Width Modulation) control scheme using FETs of the motor drive circuit as switching elements, which would however produce greater switching noise and magnetostrictive sound as the current input to the switching elements and the switching speed of the switching elements increase. The greater switching noise and magnetostrictive sound would lower the merchantability of vehicles equipped with the electric power steering apparatus. Thus, in the case of the electric power steering apparatus with two motor drive circuits, adverse influences of the switching noise and magnetostrictive sound resulting from the PWM control would become more significant because each of the two motor drive circuits produces such switching noise and magnetostrictive sound. These inconveniences commonly occur in steering apparatus including two motor drive circuits, irrespective of whether the steering assisting motors are of the brushless type or brushed type.